The field of the invention is nuclear magnetic resonance imaging (“MRI”) methods and systems. More particularly, the invention relates to producing magnetic resonance images that enable assessment of the myocardial viability in patients with coronary artery disease.
When a physician has diagnosed a patient as having ischemic heart disease, it is important to know whether the myocardium is injured or infarcted, and where. Once the existence and extent of injury and/or infarction has been determined, the physician can decide whether to treat the patient with drugs or whether to carry out a surgical intervention.
Physicians often use myocardial radionuclide studies to help make this determination. A myocardial radionuclide study is a technique whereby the patient's blood is radiolabelled using a radioisotope of a type that is taken up by myocardial tissue (e.g. Thallium). The patient's heart is then imaged using a scintillation camera in a nuclear medicine or positron emission tomography (“PET”) study. If a particular region of the myocardium takes up the radioisotope, that region is assumed to contain living tissue; if not, the region is assumed to contain infarcted tissue. However, because both perfusion and viability are necessary for uptake, it may be difficult to distinguish the relative contributions that ischemia and infarction make to the defect.
Nuclear medicine studies also have very poor spatial resolution. As a result, such studies do not precisely show where tissue is dead, where tissue is injured, and where tissue is normal. Furthermore, nuclear medicine studies may take a long time (a conventional multi-scan myocardial radionuclide study may require five hours or more including the time between scans). MRI studies, on the other hand, have excellent spatial resolution and can be completed in less than one hour.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The prevailing methods used to acquire NMR signals and reconstruct images use a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as “spin-warp”. The spin-warp technique is discussed in an article entitled “Spin-Warp NMR Imaging and Applications To Human Whole-body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one Cartesian coordinate system direction by applying a phase encoding gradient (Gy) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the sequence of “views” that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
To increase the rate at which image frames are acquired, image quality may be sacrificed by acquiring fewer phase encoding views, or by using faster pulse sequences that inherently result in lower quality images. With the spin-warp methods, therefore, there is a trade-off between the number of views that are acquired to achieve the desired image resolution and quality, and the rate at which NMR data for a complete image may be acquired.
More recently, an alternative method of acquiring NMR image data has been used in which no phase encoding gradients are employed. Instead, only a readout gradient is applied during the acquisition of each NMR signal (i.e., “view”) and a series of different views are acquired by rotating the angle of the readout gradient. Rather than sampling k-space in a rectilinear scan pattern as is done in Fourier imaging and shown in FIG. 2, this “projection reconstruction” method samples k-space with a series of views that sample radial lines extending outward from the center of k-space as shown in FIG. 3. The number of views needed to sample k-space determines the length of the scan and if an insufficient number of views are acquired, streak artifacts are produced in the reconstructed image.
Because the beating heart is constantly moving, the many different views needed to reconstruct an artifact-free image are acquired over a series of heart beats at approximately the same point in the cardiac cycle. Image acquisition is gated using an ECG trigger signal, and typically four to eight views (referred to as a “segment”) are acquired at a selected time interval after the cardiac trigger signal. The reconstructed image depicts the heart at a particular moment, or cardiac phase, in its cycle as determined by the selected delay time.
To assess myocardial viability, a number of measures can be taken to enhance the image contrast between infarcted myocardium and normal myocardium. First, a contrast agent is injected prior to image acquisition and an inversion RF pulse followed by a recovery time (TI) is performed before the acquisition of the NMR data segment. This is shown in FIG. 4, where an RF inversion pulse 10 is produced after each ECG trigger signal 12 and an NMR data segment 14 is acquired at a time interval TI thereafter by the performance of four to eight phase encodings or views 16. The contrast agent shortens the T1 relaxation value of infarcted myocardium, and as a result, the longitudinal magnetization Mz of spins in the infarcted region recovers quickly from the inversion pulse 10 as indicated by line 18. The T1 relaxation value of normal myocardium, however, is not shortened and the longitudinal magnetization Mz of normal tissue recovers from the inversion pulse 10 more slowly as indicated by curve 20. If the delay time TI is set properly, the longitudinal magnetization Mz of normal myocardium is substantially zero when the segment of NMR data 14 is acquired with the result that very little, if any NMR signals are produced by normal myocardium spins. The normal myocardium is thus suppressed in the reconstructed image. Infarcted myocardium, however, appears brightly in the image because the longitudinal magnetization 18 has recovered to a substantial amount by the time TI following the inversion pulse 10.
This contrast mechanism works well if the delay period TI is properly set. Even a few milliseconds error from the optimal TI setting shown in FIG. 4 substantially reduces the contrast between normal and infarcted myocardium in the final image. Prospectively setting the TI delay accurately is difficult because the TI delay period required to null the NMR signals from normal myocardium varies from patient to patient, and it varies as a function of the contrast agent dosage. As a result, approximately 80% of the scans must be repeated with a different TI in order to obtain a clinically acceptable image.